Gaseous fuel powered engines can operate using a range of different fuel mixtures. Different fuel mixtures may result in different quantities of pollutants such as Nitrogen Oxide and Nitrogen Dioxide (collectively referred to as NOx) being produced during combustion. Various environmental regulations have resulted in a need to significantly reduce the levels of pollutants without restricting the performance of an engine, such as measured by the maximum Brake Mean Effective Pressure (BMEP). Stoichiometric combustion can be used in some combustion engines, along with special catalysts in the exhaust aftertreatment system, to reduce levels of NOx produced during combustion. However, this may result in high combustion temperatures and increased knock propensity, which restricts the BMEP of the engine. Therefore, various alternative methods of reducing levels of pollutants such as NOx produced during combustion may include accurate control of the amounts of various constituents in the fuel, such as by controlling the amounts of H2 in the fuel. Accurate sensing and control of the composition of the fuel and of operating characteristics of the engine enables improved performance and reduced production of pollutants. Catalytic partial oxidation reforming (CPOx) is one method that may be used during operation of an engine in order to change the levels of H2 provided to the air-fuel mixture during combustion, thereby improving thermal efficiency and combustion stability.
Traditionally, a determination of the physical properties of gaseous fuel used to power combustion engines was achieved by temperature and/or pressure control of the gaseous fuel, or by means of compositional analysis such as performed using gas chromatography without control of the temperature or pressure. Methods for measuring the quality and composition of gaseous fuels enable a determination of the heating value available from a particular gaseous fuel mixture. An engine using the particular gaseous fuel mixture of determined quality and heating value can be operated by, for example, controlling ignition and fuel injection in order to maintain or improve power output and reduce production of pollutants. Some existing techniques for continuously analyzing a stream of gaseous fuel during operation of an engine use expensive measuring equipment that requires ongoing maintenance and lacks reliability under harsh field operating conditions.
One attempt to address the above-described problems is disclosed in U.S. Pat. No. 5,311,447 (the '447 patent) that issued to Bonne on May 10, 1994. In particular, the '447 patent discloses a non-combustion process for measuring the quality of fuel being fed to a gas consumption device. The method includes diverting a portion of the fuel through a sensor chamber, and measuring a viscosity of the fuel at a first sensor in the chamber. The method also includes measuring a thermal conductivity of the fuel with a second sensor in the chamber, at two different temperature levels. The viscosity and thermal conductivity values are then corrected based on a temperature and a pressure of the fuel, and a corresponding heating value is determined using an empirical formula determined as a function of the corrected viscosity and thermal conductivity values. The heating value is then stored, displayed, or given off as a control pulse depending on the information required for a particular application. The empirical formula used to calculate the heating value of the fuel is determined through the use of a commercially available regression analysis program.
Although the method described in the '447 patent may be adequate in some applications, it may be less than optimal. For example, the method relies on determination of at least two different fuel gas properties, such as viscosity and thermal conductivity, and then derivation of a characteristic of the fuel gas such as heat content using the two determined properties. As a result, the associated system requires at least two different types of sensors, adding to expense, and may have relatively slow response times dependent upon measurement of different fuel characteristics. The speed of the system may preclude its use in highly-transient applications (e.g., in combustion engine applications).
The disclosed system is directed to overcoming one or more of the problems set forth above and/or other problems associated with existing systems for determining gas compositions.